Horseshoe Harbor

Earth’s atmosphere is vital for diversity of life. Oxygen in the atmosphere developed early in earth history, before there were any complicated life forms.

How do we know this?            Stromatolites and Earth’s Life

The Copper Harbor Conglomerate is partly made up of alluvial fans. The alluvial fans are more obvious on the Keweenaw than on Isle Royale, where the sandy and silty units and more abundant and cobble sizes are generally less.  This alluvial fan hypothesis was proposed by Douglas Elmore in 1984. Many people have debated it and most support it. We make hypotheses like this to help us understand the conditions of formation of this remarkable rock formation.  There are some mind stretching implications: 1.  It implies a mountain on one side.  Where is that mountain now?  2. It suggests desert conditions. Without land plants, it is likely that the Neoproterozoic may have been a “wet” desert.

This Alluvial fan near Stovepipe wells in Death Valley CA is 9.5 km wide.

Debris flow deposits on an alluvial fan in Panamint Valley at the base of Panamint Butte in the Cottonwood Mountains in Death Valley National Park (Inyo County). Note how the stream channel is incised near the top of the fan, but the debris flow deposit is spread out on the lower fan surface.

Keweenawan Stromatolite from the Copper Harbor Conglomerate, Horseshoe Harbor.

Lake Shore Traps
Copper Harbor Conglomerate
Copper Harbor Conglomerate

Sketch from Elmore (1984), showing sedimentary features observed in the Copper Harbor beds exposed at Horseshoe Harbor.